Continental geotherm and the evolution of rifted margins

Abstract

Models of melting accompanying mantle upwelling predict far more melt than is observed at nonvolcanic margins. The discrepancy may be explained if the paradigm of a uniform asthenosphere is incorrect. Work on the velocity structure of the continents has shown that the convecting sublithospheric mantle may have a potential temperature as low as 1200 ؇C, ϳ100 ؇C cooler than that beneath the oceans. The continental geotherm derived from studies on xenoliths brought up by kimberlites is also compatible with a cool sublithospheric mantle except where perturbed by mantle plumes. Upwelling of such cool mantle during rifting leads to little melt production, even for rapid extension rates, explaining the formation of amagmatic margins away from mantle plumes. However, the transition to seafloor spreading and the development of normal-thickness oceanic crust requires the invasion of the amagmatic rift by hotter oceanic asthenosphere and/or plume material. This influx may cause a transient thermal uplift, recorded as a breakup unconformity. Conversely, at volcanic margins, the onset of seafloor spreading is accompanied by the escape of the hot plume puddle along the mid-ocean ridge system away from the volcanic margin, leading to a pulse of rapid subsidence.

The temporal and spatial distribution of volcanism in the South China Sea region

“…It is not plausible to relate the magmatism of the South China Sea region to that of Indochina by mantle flow. The paucity of volcanic rocks during pre-and syn-rift stages implies a decrease of potential temperature (Reston and Morgan, 2004), which is unfavorable for large-scale lithosphere flow.…”

Tectonostratigraphic evolution of the Labrador margin, Atlantic Canada

“…It has been suggested that these are instead related to changes in the stress field magnitude (Tucholke et al, 2007) or direction (Tankard and Welsink, 1987;Enachescu, 1987). Alternatively, Reston and Phipps-Morgan (2004) and Peron-Pinvidic et al (2007) proposed that the infiltration of hot asthenosphere below the rift, replacing cooler continental lithospheric mantle, may be responsible for uplift and erosion (see also Huismans and Beaumont, 2008).…”

Magmatic breakup as an explanation for magnetic anomalies at magma-poor rifted margins

“…Hotplume-influenced asthenosphere invading the NB and IAP rifted area has been previously suggested in ref. 25. Whether the enriched nature of the alkaline magmatism argues for a hotspot-driven origin of the Tore-Madeira rise and the J-anomaly ridge is controversial 22,27 .…”

“…21). Because large transform faults can inhibit alongaxis melt flow 24 , these ∼10 Myr could correspond to the time needed for the hot suboceanic asthenosphere to pass through the Newfoundland-Gibraltar transform faults, penetrate laterally beneath the Newfoundland-Iberia rift zone 25 and produce magma in excess that finally triggered oceanic spreading. This event of high melt delivery coincides with the opening of the southern Mid-Atlantic Ridge and with the 100-110-Myr-old change of plate velocity observed on a global scale 26 .…”